1. Field of the Invention
The present invention relates to novel zeolites based on silica and oxides of the tetravalent elements tin and zirconium, and, more especially, relates to novel zeolites based on silica and tin oxides and zeolites based on silica and zirconium oxide.
This invention also relates to novel MFI zeolites, to a process for the production thereof, and to the use of such novel MFI zeolites as catalysts in the hydroxylation of phenols or phenol ethers with hydrogen peroxide.
2. Description of the Prior Art
Zeolites are crystallized tectosilicates. Their structures are assemblies of tetrahedra TO.sub.4 which constitute a three-dimensional network by sharing oxygen atoms. In zeolites of the aluminosilicate type, which are the most typical, T represents tetravalent silicon and trivalent aluminum. The cavities and channels of molecular dimensions in this network receive cations which compensate for the charge deficit resulting from the presence of trivalent aluminum in the tetrahedra. Trivalent elements such as gallium and more rarely boron or beryllium may be substituted for the aluminum.
In general, the composition of the zeolites may be represented by the empirical formula M.sub.2/n O; Y.sub.2 O.sub.3 ; xZO.sub.2 in the dehydrated, calcined state. Z and Y respectively represent the tetravalent and trivalent elements of the tetrahedra TO.sub.4 ; M represents an electropositive element of valency n such as an alkali metal or alkaline earth metal, defining the compensating cations; x may range from 2, theoretically to infinity, in which case the zeolite is a silica.
Each type of zeolite has a distinctive porous structure. The variation in the dimensions and shapes of the pores from one type to another result in changes in the adsorbing properties thereof. Only molecules of certain dimensions and shapes can inlet the pores of a particular zeolite. These unique characteristics of the zeolites make them particularly suitable for purifying or separating mixtures of gases or liquids, for example for separating hydrocarbons by selective adsorption.
Chemical composition, particularly as regards the nature of the elements present in the tetrahedra TO.sub.4 and the nature of the exchangeable compensating cations, is also an important factor involved in the selectivity of adsorption and particularly in the catalytic properties of these materials. They are used as catalysts or catalyst carriers in cracking, reforming and modification of hydrocarbons and in the synthesis of a wide variety of chemical species.
Many zeolites are of natural origin; they are aluminosilicates, the availability and properties of which do not always meet the requirements of industrial applications. Continuing research for materials having new proprieties has, consequently, promoted the synthesis of a wide variety of zeolites, essentially of the aluminosilicate type. Exemplary thereof are zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite L (FR-A 1,224,154), zeolite T (FR-A 1,223,775), zeolite ZSM5 (U.S. Pat. No. 3,702,886), zeolite ZSM12 (U.S. Pat. No. 3,832,449) and zeolite ZSM48 (EP-A 0,015,132).
The zeolites are typically produced from a reaction mixture which is converted, in a hydrothermal medium, by a process of dissolution/recrystallization. After being separated and dried, the crystalline precipitate is calcined to provide an active zeolite.
The reaction mixture contains reagents which may provide the elements T to be incorporated in the network of the zeolite. These reagents are typically aqueous gels containing oxides or hydroxides of the elements T.
It also contains one or more mobilizing agents to promote dissolution of the reagents and transfer from the aqueous phase to the zeolite crystals which are forming, and a structuring agent which is incorporated to form microporous spaces and to stabilize the zeolite.
OH.sup.- anions are typical mobilizing agents. Thus, the reaction media are then characterized by a basic pH, generally above 10. These media are very appropriate for dissolving sources containing the elements silicon and aluminum and, in general, any elements providing oxygen-containing anions which are soluble in a basic medium.
When strong bases such as alkali metal hydroxides are used, highly supersaturated media are produced, permitting rapid crystallization of the zeolites. But it is often difficult to control the formation of the desired crystallized phase, which is metastable in many instances. On the other hand, a high rate of crystallization may result in the formation of faults within the network of the TO.sub.4 tetrahedra, such as -T-O- instead of -T-O-T-bridges. Finally, the presence of alkaline compensating cations in the channels and cavities is often disadvantageous for certain applications, and it may then be necessary to conduct an exchange with other cations. This latter disadvantage may be avoided by replacing the bases by other strong bases, such as alkyl ammonium hydroxides, which may provide a structuring function at the same time. But the high cost of these compounds limits their industrial use. The use of weaker bases such as amines, which also have structuring properties, avoids many of the disadvantages indicated above. In this event, however, the concentration of mobilizing OH.sup.- anions may become too weak, rendering the reaction speeds too low.
Moreover, it is not always easy to incorporate certain metallic species in the TO.sub.4 tetrahedra of the network of the zeolite when the reaction medium is basic.